Iridium-catalyzed 1,3-hydrogen shift/chlorination of allylic alcohols

Article abstract of DOI:10.1002/anie.201301013

Tandem: Allylic alcohols react with N-chlorosuccinimide (NCS) in a tandem 1,3-H shift/C-Cl bond formation leading to α-chloroketones and α-chloroaldehydes. The reactions proceed with complete selectivity to give single constitutional isomers of monochlorinated carbonyl compounds. The utility of the transformation is illustrated by the straightforward synthesis of 4,5-disubstituted 2-aminothiazoles from allylic alcohols. Copyright

Full text of DOI:10.1002/anie.201301013

Angewandte
Chemie
DOI: 10.1002/anie.201301013
Synthetic Methods
Iridium-Catalyzed 1,3-Hydrogen Shift/Chlorination of Allylic
Alcohols**
Nanna Ahlsten, Antonio Bermejo Gꢀmez, and Belꢁn Martꢂn-Matute*
Chlorinated compounds are among the most common and
versatile building blocks in organic synthesis. Among these, a-
chlorocarbonyl derivatives are of synthetic value owing to the
variety of functional groups that can be introduced both at the
chlorinated a-carbon atom and at the carbonyl functionality.[1]
For instance, they readily undergo substitution/addition
reactions[1,2] and cross-coupling reactions[3] and are useful
precursors to heterocycles.[4]
While a number of methods have been reported for the
electrophilic halogenation of aldehydes or ketones containing
only one enolizable position, the same reaction for unsym-
metrical aliphatic ketones is challenging.[1] Here, most
methods rely on steric or electronic differentiation for
regioselective functionalization of carbonyl compounds
(Scheme 1a).[1,5] However, many ketones lack the bias to
enolize with complete regioselectivity, and the enolization
cannot always be directed to the desired position. Impor-
tantly, the formation of mixtures of halocarbonyl compounds
limits both the yield and the overall synthetic utility owing to
the challenge of separating constitutional isomers.
Scheme 1. Synthesis of a-functionalized ketones through a) enoliza-
tion/enamine formation; b) transition-metal-catalyzed isomerization of
allylic alcohols. c) This work. LA=lewis acid; Cp*=pentamethyl cyclo-
pentadienyl; NCS=N-chlorosuccinimide.
We envisaged that a-chloroketones could be synthesized
with complete regiocontrol from allylic alcohols through
a 1,3-hydrogen shift/chlorination catalyzed by transition
metals. A considerable advantage of using allylic alcohols as
enol equivalents[6,7] is that the new bond (to the electrophile)
is formed exclusively at the alkenylic carbon atom of the
separations. Herein, we describe the first chlorination of
allylic alcohols, which affords single constitutional isomers of
a-chloroketones in up to > 99% yield, and for the first time
the formation of ketone by-products is completely suppressed
(Scheme 1c).
À
=
allylic alcohol [RCH(OH) CH CHR; Scheme 1b]. This type
We first investigated the isomerization/chlorination of
phenylpent-1-en-3-ol (2a) catalyzed by [{Cp*IrCl2}2] in the
presence of N-chlorosuccinimide (NCS). In THF and at room
temperature, only traces of the desired monochlorinated
carbonyl compound 3a were formed together with a compli-
cated mixture of by-products (Table 1, entry 1). However,
introducing water as a cosolvent had a strong influence on the
reaction outcome, and the yield of 3a gradually increased
(entries 2,3). Notably, in THF/H2O = 1:2, a quantitative yield
of 3a was obtained in 6 h with only 0.25 mol% of
[{Cp*IrCl2}2] (entry 4). Under these conditions, 3a was
formed with complete regiocontrol and nonchlorinated
ketone 4a or enone 5a were not detected. Furthermore, the
reactions do not require inert conditions. In the absence of
THF, a low yield of 3a was obtained (entry 5). Chloramine-T
or 1,3-dichloro-5,5-dimethylhydantoin afforded poor yields of
3a (entries 6–7).
of transformation has almost exclusively been investigated
using carbon electrophiles (e.g. aldehydes or imines).[8,9a,b]
A
drawback with all these procedures has always been the
undesired formation of unfunctionalized ketone by-products
(Scheme 1b). Recently, we reported the first example of 1,3-
hydrogen shift/halogenation for the preparation of a-fluoro-
ketones.[9c,d] While this represented a success in terms of
merging a transition-metal-catalyzed isomerization with an
electrophilic halogenation, the formation of nonfluorinated
ketones (5–20%) could not be avoided and led to challenging
[*] Dr. N. Ahlsten,[+] Dr. A. Bermejo Gꢀmez,[+]
Prof. Dr. B. Martꢁn-Matute
Department of Organic Chemistry, Stockholm University
10691 Stockholm (Sweden)
E-mail: belen@organ.su.se
To evaluate the reaction scope, a variety of aliphatic and
a-aryl allylic alcohols (2a–2o), including cyclic and function-
alized substrates, were subjected to the optimized reaction
conditions (Scheme 2). The corresponding a-chloroketones
(3a–3o) were obtained as single constitutional isomers in
excellent yields. Unfunctionalized ketones (4) or side prod-
ucts derived from overchlorination or from chlorination of
[+] These authors contributed equally to this work.
[**] Financial support from the Swedish Research Council (Veten-
skapsrꢂdet), the Knut and Alice Wallenberg Foundation, and the
Department of Organic Chemistry at Stockholm University is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301013.
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Table 1: Isomerization/chlorination of 2a.[a]
tion). The methodology is suitable for multigram-scale
reactions, and the chlorination of 2e and 2o gave the same
yields on a 10 g scale (52–78 mmol) as on a smaller scale
(1 mmol).
The corresponding transformation of primary allylic
alcohols into a-functionalized aldehydes is extremely
rare.[8g,10] Often, transition metal catalysts fail to promote
the tandem processes from these substrates, and side reac-
tions such as decarbonylation[11] and self-condensation[8g,12]
can occur with the aldehyde products. However, with the mild
reaction conditions reported herein, various a-chloroalde-
hydes (7a–f) were prepared in good yields from primary
allylic alcohols 6a–f (Scheme 3). In some cases, yields were
slightly lower owing to the volatility of the product (7a),
formation of enones (7b, 7d–e), or incomplete conversion
(7 f).
Entry THF/H2O Cl source
Conv. of 2a 3a/4a/5a
(v/v)
[%][b]
[%][b]
1
100:0
20:1
5:1
NCS
NCS
NCS
NCS
58
90
85
>99
34
1:–:–
2[c]
3
3:2:1
50:4:–
>99:–:–
30:–:4
4[d,e] 1:2
5
6
0:100
1:2
NCS
1,3-dichloro-5,5-di-
methylhydantoin
chloramine-T
>99
8:2:2
7
1:2
30
2:2:–
[a] 2a (0.4 mmol, 0.2m). [b] Determined by 1H NMR spectroscopy with
respect to an internal standard (1,2,4,5-tetrachloro-3-nitrobenzene).
[c] 16 h. [d] 0.25 mol% of [{Cp*IrCl2}2] (0.5 mol% Ir). [e] THF/H2O (v/v
1:1) afforded also >99% yield of 3a.
Scheme 3. Isomerization/chlorination of primary allylic alcohols. Reac-
tions were performed on a 1 mmol scale. trans-6a–e were used. Yields
of isolated products are shown (yields given in parentheses were
1
determined by H NMR spectroscopy with respect to 1,2,4,5-tetra-
chloro-3-nitrobenzene as internal standard).
The varied results obtained in different mixtures of THF
and H2O (Table 1) indicate the need for a highly polar solvent
and may be related to catalyst activation through dissociation
of a chloride ligand from 1. A comparison of the catalytic
activity of 1 with that of cationic [Cp*Ir(H2O)3]SO4 (8)[13] in
the chlorination of 2a (THF/H2O = 1:2) resulted in identical
reaction profiles. This result suggests that 1 and 8 are equally
capable of forming an active catalyst at this THF/H2O ratio
(Figure 1 and Figure S1 in the Supporting Information).
However, by progressively decreasing the amount of water
in the solvent, significant differences between 1 and 8 were
observed. Thus, while severely diminished yields were
obtained with 1 (Figure 1, middle row), the yields with aqua
complex 8 were unaffected even at a THF/H2O ratio of 20:1
(back row). Furthermore, addition of NaCl to 8 gave results
comparable to those obtained with 1 (front vs. middle row).
The requirement for an aqueous solvent (Table 1) is therefore
likely associated with chloride dissociation from 1. Still, water
cannot be fully excluded from the solvent even with the
Scheme 2. Isomerization/chlorination of sec-allylic alcohols. Reactions
were performed on a 1 mmol scale. trans-2 f, 2g, and 2h were used.
Yields of isolated products are shown (yields given in parentheses
were determined by 1H NMR spectroscopy with respect to 1,2,4,5-
tetrachloro-3-nitrobenzene as internal standard). [a] THF/H2O (v/v
1:2). [b] 0.25 mol% of [{Cp*IrCl2}2]. [c] THF/H2O (v/v 1:1).
[d] 0.5 mol% of [{Cp*IrCl2}2].
benzylic positions were not detected for any of the substrates.
The enantiopure allylic alcohol 2k underwent the chlorina-
tion reaction without epimerization of the stereogenic center.
Allylic alcohols with trisubstituted or 1,1-disubstituted olefins
did not give satisfactory yields (see the Supporting Informa-
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show here a short synthesis of 2-aminothiazoles (9–12) from
allylic alcohols (2 f, 2c, 2o, and 2e, respectively, Scheme 6).
The straightforward and high-yielding reactions illustrate the
usefulness of this procedure in the preparation of target
compounds that rely on a-chlorocarbonyl compounds as
intermediates.
Figure 1. Reaction of 2a with NCS in THF/H2O mixtures after 4.5 h
with 8 (back), 1 (middle), and 8/NaCl (5 mol%) (front). The yield with
1 in THF/H2O=5:1 is the average of four reactions.
chloride-free complex 8; in pure THF 8 afforded 3a in only
40% yield together with 22% of the nonchlorinated ketone
4a (back row).[14]
Scheme 6. Synthesis of 2-aminothiazoles from allylic alcohols. For
details see the Supporting Information. Yields of isolated products
over 2 steps are shown.
Previous studies support that the formation of a-function-
alized carbonyl compounds from allylic alcohols occurs
through: 1) a transition-metal-catalyzed 1,3-hydrogen shift
forming an enolate or an enol intermediate, and 2) subse-
quent nucleophilic attack on the electrophile (Scheme 4).[6–9]
In a cross-over experiment using deuterium-labeled [D1]-2 f,
we have confirmed that the isomerization follows an intra-
molecular 1,3-hydrogen shift (Scheme 5 and the Supporting
In conclusion, we report the first synthesis of a-chlori-
nated carbonyl compounds from secondary and primary
allylic alcohols. Awide range of substrates were chlorinated in
high yields and as single constitutional isomers. For the first
time, the formation of nonfunctionalized ketone by-products
has been suppressed. The reactions are air-tolerant, run in
water/organic solvent mixtures at room temperature, and
require low loadings of iridium. The methodology is opera-
tionally very simple and can be scaled up. On-going mech-
anistic investigations should also contribute to the future
development of Ir-catalyzed reactions for the formation of
carbon–heteroatom bonds, and will be reported in due course.
Scheme 4. Transformation of allylic alcohols via enol intermediates.
Experimental Section
[{Cp*IrCl2}2] (1, 0.25–0.5 mol%) was dissolved in THF/H2O (4.8 mL;
1:1 or 1:2). The allylic alcohol (1 mmol, 1 equiv) and NCS (1.2 equiv)
were added, and the reaction was stirred at room temperature until
full conversion. The mixture was extracted with Et2O (3 ꢀ 1 mL) and
the organic layers dried over MgSO4 and evaporated. Purification by
silica column chromatography (pentane/Et2O) afforded the a-chlori-
nated ketone/aldehyde.
Scheme 5. Cross-over and deuterium labeling experiment (see the
Supporting Information).
Information).[7b,8] Although this result is consistent with the
intermediacy of enol(ate)s, the details of the mechanism, and
Received: February 4, 2013
Published online: April 24, 2013
À
in particular of the C Cl bond formation step, remain to be
elucidated.
Keywords: allylic alcohols · chlorination · hydrogen transfer ·
To demonstrate the potential of the present methodology,
the procedure was used as a key step in the synthesis of 2-
aminothiazoles. These are privileged structures that have
found pharmaceutical applications such as in antibiotics[15]
and anti-inflammatory drugs.[16] Despite their straightforward
synthesis by condensation of thiourea with the corresponding
a-chlorocarbonyl compounds, only a few 4,5-disubstituted 2-
aminothiazoles have been reported.[17] We reasoned that the
substitution pattern of these compounds could easily be
varied by using a synthetic route from allylic alcohols that
gives access to the appropriate chloroketone precursor. We
.
iridium · isomerization
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